The present invention relates to a device for heat-sealing a tubular packaging material filled with a liquid beverage or like contents transversely thereof at a lengthwise spacing corresponding to a single container.
Heat sealing devices are already known for use with a tubular packaging material which is made of a heat-sealable paper-base laminate having an aluminum foil layer and which is to be cut transversely thereof into lengths. The device comprises a U-shaped high-frequency coil provided in a pressing surface of one of a pair of press members and having two straight portions extending in parallel to each other for heating the material at opposite sides of and along each of the portions thereof to be cut.
To effectively utilize the lines of magnetic force generated by the high-frequency coil by concentrating the lines on the inner side of the two straight coil portions, an arrangement is also known which has a magnetic member disposed over the entire inner side of each straight portion or locally on the inner side thereof (Examined Japanese Patent Publication No. 3215/1975 and Unexamined Japanese Patent Publication No. 52025/1987).
Another device of the type described is also known wherein each of the two straight coil portions is provided with a horizontal ridge extending approximately over the entire length thereof for excluding extraneous matter (Unexamined Japanese Patent Publication No. 134744/1983).
The device wherein the magnetic member is provided has the following problem. With reference to FIG. 10, beads 52 of molten resin are produced in an undulating form at a seal portion 51 of a tubular packaging material T along the edge thereof inside the container to be formed. If the container having such beads 52 is subjected to an external force, a crack will develop from the crest portion 53 of the bead 52 to break the container. Formation of such beads 52 are thought attributable to the following cause. Although the magnetic member disposed on the inner side of each of two straight portions serves to effectively utilize lines of magnetic force, the member fails to prevent lines of magnetic force from spreading outward from the straight portion, consequently permitting the material T to be heated to an unnecessary temperature at the portion thereof outside the straight portion.
With the device having the horizontal ridge, the ridge presses the seal portion with an increased pressure over the entire length thereof but is unable to press the seal portion uniformly over the entire length because the seal portion of the tube is not uniform in the thickness of the packaging material over the entire length. The tube has a seam at its center portion and a joint of the front and rear sides of the tube at each edge thereof. Especially, the seam of the tube is a lap having three times the thickness of the packaging material and has a larger thickness than the other portion, with the result that the seam is pressed under a greater pressure than is needed, permitting the paper layer of the packaging material to crack at this portion.